Vision-guided alignment method

ABSTRACT

A method for performing an operation on an article, the article including a plurality of fiducial marks and a set of features. A digital imaging system is used to capture a digital image of the article, which is analyzed to determine spatial relationships between positions of the features and positions of the fiducial marks. The article is positioned in proximity to an instrument and a fiducial sensing system including a plurality of fiducial sensors is used to determine positions of the fiducial marks. Predicted positions of the features are determined responsive to the determined spatial relationships and the determined fiducial mark positions. The position of the instrument or the article is adjusted responsive to the predicted positions of the features, and the instrument is used to perform an operation on the article.

CROSS-REFERENCE TO RELATED APPLICATIONS

Reference is made to commonly-assigned, co-pending U.S. patentapplication Ser. No. ______ (K001870), entitled “Electrical test systemwith vision-guided alignment” by E. Zeise et al.; to commonly-assigned,co-pending U.S. patent application Ser. No. ______ (K001876), entitled“Electrical test method with vision-guided alignment” by E. Zeise etal.; and to commonly-assigned, co-pending U.S. patent application Ser.No. ______ (K001877), entitled “Vision-guided alignment system” by E.Zeise et al., each of which is incorporated herein by reference.

FIELD OF THE INVENTION

This invention pertains to the field of alignment of an instrumentrelative to an article for performing an operation, and moreparticularly to vision-guided alignment of the position of theinstrument responsive to predicted positions of features of the article.

BACKGROUND OF THE INVENTION

Touch screens are visual displays with areas that may be configured todetect both the presence and location of a touch by, for example, afinger, a hand or a stylus. Touch screens may be found in televisions,computers, computer peripherals, mobile computing devices, automobiles,appliances and game consoles, as well as in other industrial, commercialand household applications. A capacitive touch screen includes asubstantially transparent substrate which is provided with electricallyconductive patterns that do not excessively impair thetransparency—either because the conductors are made of a material, suchas indium tin oxide, that is substantially transparent, or because theconductors are sufficiently narrow that the transparency is provided bythe comparatively large open areas not containing conductors. Forcapacitive touch screens having metallic conductors, it is advantageousfor the features to be highly conductive but also very narrow.Capacitive touch screen sensor films are an example of an article havingvery fine features with improved electrical conductivity resulting froman electroless plated metal layer.

Projected capacitive touch technology is a variant of capacitive touchtechnology. Projected capacitive touch screens are made up of a matrixof rows and columns of conductive material that form a grid. Voltageapplied to this grid creates a uniform electrostatic field, which can bemeasured. When a conductive object, such as a finger, comes intocontact, it distorts the local electrostatic field at that point. Thisis measurable as a change in capacitance. The capacitance can bemeasured at every intersection point on the grid. In this way, thesystem is able to accurately track touches. Projected capacitive touchscreens can use either mutual capacitive sensors or self capacitivesensors. In mutual capacitive sensors, there is a capacitor at everyintersection of each row and each column. A 16×14 array, for example,would have 224 independent capacitors. A voltage is applied to the rowsor columns. Bringing a finger or conductive stylus close to the surfaceof the sensor changes the local electrostatic field which reduces themutual capacitance. The capacitance change at every individual point onthe grid can be measured to accurately determine the touch location bymeasuring the voltage in the other axis. Mutual capacitance allowsmulti-touch operation where multiple fingers, palms or styli can beaccurately tracked at the same time.

WO 2013/063188 by Petcavich et al. discloses a method of manufacturing acapacitive touch sensor using a roll-to-roll process to print aconductor pattern on a flexible transparent dielectric substrate. Afirst conductor pattern is printed on a first side of the dielectricsubstrate using a first flexographic printing plate and is then cured. Asecond conductor pattern is printed on a second side of the dielectricsubstrate using a second flexographic printing plate and is then cured.The ink used to print the patterns includes a catalyst that acts as seedlayer during subsequent electroless plating. The electrolessly platedmaterial (e.g., copper) provides the low resistivity in the narrow linesof the grid needed for excellent performance of the capacitive touchsensor. Petcavich et al. indicate that the line width of theflexographically printed material can be 1 to 50 microns.

Flexography is a method of printing or pattern formation that iscommonly used for high-volume printing runs. It is typically employed ina roll-to-roll format for printing on a variety of soft or easilydeformed materials including, but not limited to, paper, paperboardstock, corrugated board, polymeric films, fabrics, metal foils, glass,glass-coated materials, flexible glass materials and laminates ofmultiple materials. Coarse surfaces and stretchable polymeric films arealso economically printed using flexography.

Flexographic printing members are sometimes known as relief printingmembers, relief-containing printing plates, printing sleeves, orprinting cylinders, and are provided with raised relief images ontowhich ink is applied for application to a printable material. While theraised relief images are inked, the recessed relief “floor” shouldremain free of ink.

Although flexographic printing has conventionally been used in the pastfor printing of images, more recent uses of flexographic printing haveincluded functional printing of devices, such as touch screen sensorfilms, antennas, and other devices to be used in electronics or otherindustries. Such devices typically include electrically conductivepatterns.

To improve the optical quality and reliability of the touch screensensor film, it has been found to be preferable that the width of thegrid lines be approximately 2 to 10 microns, and even more preferably tobe 4 to 8 microns. In addition, in order to be compatible with thehigh-volume roll-to-roll manufacturing process, it is preferable for theroll of flexographically printed material to be electroless plated in aroll-to-roll electroless plating system.

After the touch screen sensor film has been printed and plated inroll-to-roll format, electrical testing is typically performed toeliminate defective touch screen sensor devices. For compatibility withhigh-volume roll-to-roll manufacturing processes, it is preferable toelectrically test the touch screen sensor devices while they are stillin roll format and prior to separation from the roll. This isparticularly true if there are subsequent roll-to-roll processes such asapplication of protective liners or films.

The use of machine vision systems for aligning test probes with testpads of a device to be electrically tested is known. For example, U.S.Pat. No. 5,442,299 to Caggiano, entitled “Printed circuit board testfixture and method,” and U.S. Pat. No. 5,321,351 to Swart et al.,entitled “Test fixture alignment system,” disclose test systems forprinted circuit boards where alignment of the test probes is performedwith reference to the sensed positions of fiducial marks formed on theprinted circuit board.

With increased emphasis on device miniaturization it is desired toreduce the size and spacing of test pads, thereby requiring tighteralignment tolerances for the test probes. In addition, for roll-to-rollfabrication of devices on a flexible web of substrate, there can bedistortion due to web tension or slippage during printing for example.The distortion can cause errors in the placement of the fiducial marksrelative to the test pads. For a large area device or a large area setof devices being tested simultaneously, such placement errors of thefiducial marks can decrease the reliability of test probe alignment withthe test pads, especially if the test pads are small and closely spaced.A test probe that misses its pad can result in the erroneous detectionof an open circuit such that a false failure is recorded.

What is needed is a machine vision system that is capable of reliablealignment of test probes of an electrical test fixture to test pads of adevice, even if there is distortion in the placement of fiducial marksrelative to test pads.

Alignment of test probes for roll-to-roll electrical testing is anexample of a more general problem for systems configured to perform anoperation on an article, where the article includes a plurality offiducial marks and a set of features of interest. The problem is that iffiducial marks are provided for determining the positions of the set offeatures, and if there is a variable placement error between thefiducial marks and the set of features, then the determination of thelocation of the fiducial marks alone can provide unreliable informationregarding the positions of the set of features. The subsequentlyperformed operation can sometimes miss its intended position when thevariable placement error is sufficiently large. What is needed is asystem that is capable of accurate determination of the positions offeatures of interest, even if there is uncertainty in the placementaccuracy of fiducial marks associated with the features of interest.

SUMMARY OF THE INVENTION

The present invention represents a method for using an instrument toperform an operation on an article, the article including a plurality offiducial marks and a set of features, comprising:

a) positioning the article in proximity to a digital imaging system;

b) using the digital imaging system to capture a digital image of atleast a portion of the article that includes the set of features and theplurality of fiducial marks;

c) automatically analyzing the captured digital image to determinepositions of the plurality of fiducial marks and the set of featureswithin the captured digital image;

d) determining spatial relationships between the determined positions ofthe plurality of fiducial marks and the determined positions of the setof features in the captured digital image;

e) positioning the article in proximity to the instrument;

f) using a fiducial sensing system to sense positions of the pluralityof fiducial marks;

g) determining predicted positions of the features responsive to thedetermined positions of the plurality of fiducial marks and thedetermined positions of the set of features in the captured digitalimage and the sensed positions of the plurality of fiducial marks;

h) adjusting a position of the instrument or the article responsive tothe predicted positions of the features; and

i) controlling the instrument to perform the operation on the article.

This invention has the advantage that the instrument can be moreaccurately positioned relative to the article features to compensate forany deviation of the article features from their nominal positions,thereby improving the reliability of the operation performed on thearticle.

It has the additional advantage that accurate alignment is possible evenwhen there are variable placement errors between the fiducial marks andthe article features.

It has the further advantage that the accurate estimates of thepositions of the article features can be determined even when it is notpractical to position a digital imaging system to capture images of thearticle features when the article is positioned in proximity to theinstrument.

It has the still further advantage of providing a means for calibratingthe digital imaging system.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic side view of a flexographic printing system forroll-to-roll printing on both sides of a substrate;

FIG. 2 is a high-level system diagram for an apparatus having a touchscreen with a touch sensor that can be printed using embodiments of theinvention;

FIG. 3 is a side view of the touch sensor of FIG. 2;

FIG. 4 is a top view of a conductive pattern printed on a first side ofthe touch sensor of FIG. 3;

FIG. 5 is a top view of a conductive pattern printed on a second side ofthe touch sensor of FIG. 3;

FIG. 6 is a top view of a touch sensor formed on a transparent web ofsubstrate;

FIG. 7 is a lower magnification top view of the web of substrate of FIG.6 showing three successive frames of touch sensors;

FIG. 8 is a schematic side view of an exemplary roll-to-roll electricaltest system;

FIG. 9 is similar to the top view of FIG. 7, but also shows the positionof the electrical test fixture relative to the web of substrate;

FIG. 10 is a schematic side view of a roll-to-roll electrical testsystem according to an embodiment of the invention;

FIG. 11 is similar to the top view of FIG. 7, but also shows the captureof image lines by a line-scan camera;

FIG. 12 is a flow diagram for a method of electrically testing devicesaccording to an embodiment of the invention;

FIG. 13 is a top view of a frame containing multiple touch sensors inthe same orientation;

FIG. 14 is similar to FIG. 13, except that two of the touch sensors arerotated by 180 degrees;

FIG. 15A illustrates coupling the device with the test result byproviding a mark near the device;

FIG. 15B illustrates coupling the device with the test result using anidentification code for the device;

FIG. 16 shows an example of an article including fuses on which a fuseblowing operation is to be performed;

FIG. 17 is a schematic side view of a roll-to-roll system for performingan operation on an article according to an embodiment of the invention;

FIG. 18 is a flow diagram for a generalized method of performing anoperation on an article according to an embodiment of the invention; and

FIG. 19 shows an example of an article including a resistor array onwhich a material removal operation is to be performed for resistortrimming.

It is to be understood that the attached drawings are for purposes ofillustrating the concepts of the invention and may not be to scale.Identical reference numerals have been used, where possible, todesignate identical features that are common to the figures.

DETAILED DESCRIPTION OF THE INVENTION

The present description will be directed in particular to elementsforming part of, or cooperating more directly with, an apparatus inaccordance with the present invention. It is to be understood thatelements not specifically shown, labeled, or described can take variousforms well known to those skilled in the art. In the followingdescription and drawings, identical reference numerals have been used,where possible, to designate identical elements. It is to be understoodthat elements and components can be referred to in singular or pluralform, as appropriate, without limiting the scope of the invention.

The invention is inclusive of combinations of the embodiments describedherein. References to “a particular embodiment” and the like refer tofeatures that are present in at least one embodiment of the invention.Separate references to “an embodiment” or “particular embodiments” orthe like do not necessarily refer to the same embodiment or embodiments;however, such embodiments are not mutually exclusive, unless soindicated or as are readily apparent to one of skill in the art. Itshould be noted that, unless otherwise explicitly noted or required bycontext, the word “or” is used in this disclosure in a non-exclusivesense.

The example embodiments of the present invention are illustratedschematically and may not be to scale for the sake of clarity. One ofordinary skill in the art will be able to readily determine the specificsize and interconnections of the elements of the example embodiments ofthe present invention.

References to upstream and downstream herein refer to direction of flow.Web media moves along a web transport path in a web advance directionfrom upstream to downstream.

As described herein, exemplary embodiments of the present inventionprovide a vision-guided roll-to-roll electrical testing system andmethod where an electrical test fixture is aligned to test pads formedon a web of substrate using a digital imaging system for capturing adigital image of at least the test pads of a particular device plusassociated fiducial marks, and using a separate fiducial sensing systemfor sensing positions of the associated fiducial marks. Broaderapplications of position determination of features using a digitalimaging system plus a separate fiducial sensing system are alsocontemplated as described below. Within the context of the presentinvention, the term “vision-guided” refers to the use of a digitalimaging system (i.e., a computer vision system) to guide the alignmentof an instrument (e.g., an electrical test fixture) with an object(e.g., an electrical device).

A context for roll-to-roll electrical testing will be provided relativeto flexographically printed touch sensor films for touch screens. FIG. 1is a schematic side view of a flexographic printing system 100 that canbe used in embodiments of the invention for roll-to-roll printing oftouch sensor films on both sides of a substrate 150. Substrate 150 isfed as a web from supply roll 102 to take-up roll 104 throughflexographic printing system 100. Substrate 150 has a first side 151 anda second side 152. Optionally, the printed features can subsequently beelectroless plated for improved electrical conductivity

The flexographic printing system 100 includes two print modules 120 and140 that are configured to print on the first side 151 of substrate 150,as well as two print modules 110 and 130 that are configured to print onthe second side 152 of substrate 150. The web of substrate 150 travelsoverall in roll-to-roll direction 105 (left to right in the example ofFIG. 1). However, various rollers 106 and 107 are used to locally changethe direction of the web of substrate as needed for adjusting webtension, providing a buffer, and reversing the substrate 150 forprinting on an opposite side. In particular, note that in print module120 roller 107 serves to reverse the local direction of the web ofsubstrate 150 so that it is moving substantially in a right-to-leftdirection.

Each of the print modules 110, 120, 130, 140 includes some similarcomponents including a respective plate cylinder 111, 121, 131, 141, onwhich is mounted a respective flexographic printing plate 112, 122, 132,142, respectively. Each flexographic printing plate 112, 122, 132, 142has raised features 113 defining an image pattern to be printed on thesubstrate 150. Each print module 110, 120, 130, 140 also includes arespective impression cylinder 114, 124, 134, 144 that is configured toforce a side of the substrate 150 into contact with the correspondingflexographic printing plate 112, 122, 132, 142. Impression cylinders 124and 144 of print modules 120 and 140 (for printing on first side 151 ofsubstrate 150) rotate counter-clockwise in the view shown in FIG. 1,while impression cylinders 114 and 134 of print modules 110 and 130 (forprinting on second side 152 of substrate 150) rotate clockwise in thisview.

Each print module 110, 120, 130, 140 also includes a respective aniloxroller 115, 125, 135, 145 for providing ink to the correspondingflexographic printing plate 112, 122, 132, 142. As is well known in theprinting industry, an anilox roller is a hard cylinder, usuallyconstructed of a steel or aluminum core, having an outer surfacecontaining millions of very fine dimples, known as cells. Ink isprovided to the anilox roller by a tray or chambered reservoir (notshown). In some embodiments, some or all of the print modules 110, 120,130, 140 also include respective UV curing stations 116, 126, 136, 146for curing the printed ink on substrate 150.

FIG. 2 shows a high-level system diagram for an apparatus 500 having atouch screen 510 including a display device 520 and a touch sensor 530that overlays at least a portion of a viewable area of display device520. Touch sensor 530 senses touch and conveys electrical signals(related to capacitance values for example) corresponding to the sensedtouch to a touch screen controller 580. Touch sensor 530 is an exampleof an article that can be tested using a roll-to-roll electrical testsystem described below.

FIG. 3 shows a schematic side view of a touch sensor 530. Transparentsubstrate 540, for example polyethylene terephthalate, has a firstconductive pattern 550 formed on a first side 541, and a secondconductive pattern 560 formed on a second side 542.

FIG. 4 shows an example of a conductive pattern 550 that can be printedon first side 541 (FIG. 3) of substrate 540 (FIG. 3) using one or moreprint modules such as print modules 120 and 140 (FIG. 1) of flexographicprinting system 100 (FIG. 1) as described above. Conductive pattern 550includes a grid 552 including grid columns 555 of intersecting finelines 551 and 553 that are connected to an array of channel pads 554.Interconnect lines 556 connect the channel pads 554 to the connectorpads 558 that are connected to touch screen controller 580 (FIG. 2).Conductive pattern 550 can be printed by a single print module 120 insome embodiments. However, because the optimal print conditions for finelines 551 and 553 (e.g., having line widths on the order of 4 to 8microns) are typically different than for printing the wider channelpads 554, connector pads 558 and interconnect lines 556, it can beadvantageous to use one print module 120 for printing the fine lines 551and 553 and a second print module 140 for printing the wider features.Furthermore, for clean intersections of fine lines 551 and 553, it canbe further advantageous to print and cure one set of fine lines 551using one print module 120, and to print and cure the second set of finelines 553 using a second print module 140, and to print the widerfeatures using a third print module (not shown in FIG. 1) configuredsimilarly to print modules 120 and 140.

FIG. 5 shows an example of a conductive pattern 560 that can be printedon second side 542 (FIG. 3) of substrate 540 (FIG. 3) using one or moreprint modules such as print modules 110 and 130 (FIG. 1) of flexographicprinting system 100 (FIG. 1) as described above. Conductive pattern 560includes a grid 562 including grid rows 565 of intersecting fine lines561 and 563 that are connected to an array of channel pads 564.Interconnect lines 566 connect the channel pads 564 to the connectorpads 568 that are connected to touch screen controller 580 (FIG. 2). Insome embodiments, conductive pattern 560 can be printed by a singleprint module 110. However, because the optimal print conditions for finelines 561 and 563 (e.g., having line widths on the order of 4 to 8microns) are typically different than for the wider channel pads 564,connector pads 568 and interconnect lines 566, it can be advantageous touse one print module 110 for printing the fine lines 561 and 563 and asecond print module 130 for printing the wider features. Furthermore,for clean intersections of fine lines 561 and 563, it can be furtheradvantageous to print and cure one set of fine lines 561 using one printmodule 110, and to print and cure the second set of fine lines 563 usinga second print module 130, and to print the wider features using a thirdprint module (not shown in FIG. 1) configured similarly to print modules110 and 130.

Alternatively, in some embodiments conductive pattern 550 can be printedusing one or more print modules configured like print modules 110 and130, and conductive pattern 560 can be printed using one or more printmodules configured like print modules 120 and 140 of FIG. 1.

With reference to FIGS. 2-5, in operation of touch screen 510, touchscreen controller 580 can sequentially electrically drive grid columns555 via connector pads 558 and can sequentially sense electrical signalson grid rows 565 via connector pads 568. In other embodiments, thedriving and sensing roles of the grid columns 555 and the grid rows 565can be reversed.

FIG. 6 is a top view of a touch sensor 530 formed on a transparent webof substrate 150 together with fiducial marks 531 and 532 near a firstedge 543 and fiducial marks 533 and 534 near a second edge 544. Fiducialmarks 531, 532, 533 and 534 are formed at predetermined target positionsrelative to a target position of the associated touch sensor 530. Grid552 (FIG. 4) formed on first side 541 and grid 562 (FIG. 5) formed onsecond side 542 (FIG. 3) are represented together as a shaded region inFIG. 6. On first side 541, connector pads 558 are connected to columnchannel pads 554 by interconnect lines 556 and are connected to probepads 559 through grid 552. On second side 542, connector pads 568 areconnected to row channel pads 564 by interconnect lines 566 and areconnected to probe pads 569 through grid 562. (For clarity, only asubset of the interconnect lines 556 and 566 are shown.) In someembodiments some of the fiducial marks such as fiducial marks 531 and532 are formed on the first side 541 of the substrate 150 and otherfiducial marks such as fiducial marks 533 and 534 are formed on thesecond side 542 (FIG. 3) of the substrate 150.

Touch sensor 530 and fiducial marks 531, 532, 533 and 534 are formedwithin a frame 300 of substrate 150 defined between two frame boundaries546, as well as first and second edges 543, 544. FIG. 7 is similar toFIG. 6 but shows a lower magnification top view of web of substrate 150so that three successive frames 300 separated by frame boundaries 546are shown. Each frame 300 includes a touch sensor 530 and the associatedfiducial marks 531, 532, 533 and 534. In the example shown in FIG. 7,fiducial marks 531 and 532 are formed such that their center-to-centerpositions are a spacing D1 apart.

Returning to a discussion of FIG. 6, in order to determine whether aparticular touch sensor 530 has any electrical defects an electricaltest is performed to test continuity or resistance between connectorpads 558 and corresponding probe pads 559 on first side 541 of substrate150 and between connector pads 568 and corresponding probe pads 569 onsecond side 542 (FIG. 3) of substrate 150, looking for electrical shortsand opens. Herein the term “test pads” will be used generically toinclude connector pads 558, 568 as well as probe pads 559, 569.

FIG. 8 shows a schematic side view of a basic roll-to-roll electricaltest system 200. Web of substrate 150 is guided by rollers 206 and 207from a supply roll 202 to a take-up roll 204 along a web transport path205. Rollers 206 and 207 are part of a web transport system, which alsoincludes at least one motor (not shown) to advance the web of substrate150 with an appropriate amount of tension. Web of substrate 150 isguided to electrical test fixture 220 having an upper test bed 221having upper test probes 222 for contacting a first set of test pads(connector pads 558 and probe pads 559) on the first side 151 of web 150and a lower test bed 223 with lower test probes 224 for contacting asecond set test pads (connector pads 568 and probe pads 569) on thesecond side 152 of web 150. As described below, electrical test fixture220 is movable so that its position and orientation can be adjusted tobring the test probes 222 and 224 into aligned contact with theconnector pads 558, 568 and probe pads 559, 569.

A closed position of electrical test fixture 220 for performing theelectrical test using test instrument 240 is shown in FIG. 8 where theupper test probes 222 and lower test probes 224 are positioned incontact with the respective test pads. Electrical test fixture 220 alsohas an open position (not shown) where the upper test probes 222 andlower test probes 224 are withdrawn from contacting the test pads forallowing web of substrate 150 to be advanced for testing the touchsensor 530 (FIG. 7) in the next frame 300 (FIG. 7). Electrical testfixture 220 can be opened, for example by vertically moving the uppertest bed 221 and the lower test bed 223 away from the web of substrate150, or by pivoting one or both of the upper test bed 221 and the lowertest bed 223 about a hinge to open like a clamshell.

When web of substrate 150 has been advanced such that frame 300 islocated at electrical test fixture 220, clamps 245 on either side ofelectrical test fixture 220 fix the web of substrate 150 in a stoppedposition, where the web of substrate is clamped against rollers 207. Inthe example shown in FIG. 8, a fiducial sensing system 250, including afirst fiducial sensor 251 and a second fiducial sensor 252, is providedfor viewing the web of substrate 150 at the approximate positions offiducial marks 531, 532 (FIG. 9) in the frame 300 that is positioned atthe electrical test fixture 220. First fiducial sensor 251 and secondfiducial sensor 252 can include a photodiode, a photodiode array, a CMOSsensor, a charge coupled device, a digital camera, or any other type ofsensing device known in the art that can sense the fiducial marks 531,532. First fiducial sensor 251 and second fiducial sensor 252 have acenter-to-center spacing D2, which is nominally the same as the targetspacing D1 between the fiducial marks 531 and 532 in FIG. 7. (While thespacing D1 between fiducial marks 531 and 532 is ideally equal to D2,manufacturing tolerances can cause D1 to vary somewhat.)

A controller 260 determines the positions of the fiducial marks 531, 532using fiducial sensing system 250. Based on the known target spatialrelationships between the touch sensor 530 and the fiducial marks 531,532, the controller 260 adjusts a position of the electrical testfixture 220 such that the test probes 222 should be aligned with thecorresponding test pads (connector pads 558 and probe pads 559) on thefirst side 151, and the test probes 224 should be aligned with thecorresponding test pads (connector pads 568 and probe pads 569) on thesecond side 152.

With reference to FIG. 9, adjustment of a position of the electricaltest fixture 220 (FIG. 8) can include translating the electrical testfixture 220 within an x-y plane parallel to a surface of the web ofsubstrate 150. Adjustment of the position of the electrical test fixture220 can also include rotating the electrical test fixture 220 by anangle θ about an axis that is perpendicular to a surface of the web ofsubstrate 150. The electrical test fixture 220 is then closed to bringthe test probes 222, 224 into contact with the web of substrate 150, andthe controller causes an electrical test of the particular device to beperformed using a test instrument 240.

FIG. 9 is similar to the top view of FIG. 7, but instead of showing thetouch sensor 530 in the rightmost frame 300, the positions of upper testprobes 222 a, 222 b and lower test probes 224 a, 224 b of electricaltest fixture 220 are shown in order to illustrate the probeconfiguration relative to the positions of connector pads 558, 568 andprobe pads 559, 569 that are shown for the touch sensor 530 in thecenter frame 300. Upper test probes 222 a correspond to connector pads558 and upper test probes 222 b correspond to probe pads 559 on firstside 151 (FIG. 8) of web of substrate 150. Lower test probes 224 acorrespond to connector pads 568 and lower test probes 224 b correspondto probe pads 569 on second side 152 (FIG. 8) of web of substrate 150.In FIG. 9, web of substrate 150 has been advanced along media advancedirection 208 until the rightmost frame 300 of FIG. 7 is located atelectrical test fixture 220.

Also shown in the rightmost frame in FIG. 9 is a first field-of-view 253associated with first fiducial sensor 251 (FIG. 8) and a secondfield-of-view 254 associated with second fiducial sensor 252 (FIG. 8).Note that the fields-of-view 253, 254 of the fiducial sensors 251, 252are typically larger than the fiducial marks 531, 532. In general, thefields-of-view 253, 254 of the fiducial sensors 251, 252 should belarger than an expected variability in the position of the correspondingfiducial marks 531, 532. This enables the detection of the fiducialmarks 531, 532 as their position varies. Part of the variability in theposition can result from positioning tolerances of the web of substrate150 relative to electrical test fixture 220 as the web of substrate 150is advanced and then stopped and clamped. Another part of thevariability in the position can result from fabrication tolerancesduring formation of the fiducial marks 531, 532, for example due to webstretching or slipping. Depending upon fiducial geometry and sensorconfiguration, in some embodiments (not shown) the fields-of-view 253,254 need not be larger than entire corresponding fiducial mark. Forexample, for a fiducial mark having a cross-hair shape, the criticalpositioning information is contained at the position where the two armsof the cross-hair intersect, not at the extreme ends of the arms.

Satisfactory operation of the exemplary roll-to-roll electrical testsystem 200 shown in FIG. 8 depends upon there being sufficiently smallfabrication errors in relative positions between the fiducial marks 531,532 and the test pads (connector pads 558, 568 and probe pads 559, 569)within a given frame 300. As shown in FIG. 9, the fields-of-view 253 and254 are far smaller than the entire frame 300. The positions of the testpads (connector pads 558, 568 and probe pads 559, 569) are estimatedbased on the ideal spatial relationships between the test pads and thefiducial marks, 531, 532, as well as the actual positions of thefiducial marks 531, 532 as measured by the fiducial sensors 251, 252,respectively. While the fiducial sensing system 250 (FIG. 8) candetermine actual positions of fiducial marks 531, 532 with a high degreeof accuracy, fabrication-related distortion can produce errors in theestimated positions of the connector pads 558, 568 and probe pads 559,569. This can be especially problematic where the connector pads 558,568 or probe pads 559, 569 are small and spaced closely together, or ifthe connector pads 558, 568 or probe pads 559, 569 are locatedrelatively far from the fiducial marks 531, 532. In such situations thetest probes 222 a, 222 b, 224 a, 224 b can be sufficiently misalignedsuch that false “electrical opens” can be detected because a particulartest probe misses a corresponding test pad, or false “electrical shorts”can be detected because a particular test probe bridges between twoadjacent test pads.

FIG. 10 shows a schematic side view of an exemplary improvedroll-to-roll electrical test system 210 according to an embodiment ofthe invention. The primary difference between the improved roll-to-rollelectrical test system 210 and basic roll-to-roll electrical test system200 (FIG. 8) is the incorporation of a digital imaging system 270 in theelectrical test system 210 for capturing a digital image of at least aportion of the frame 300 (FIG. 7) that includes the test pads (i.e.,connector pads 558, 568 and probe pads 559, 569) for the touch sensor530, as well as the associated fiducial marks 531, 532. In the exampleshown in FIG. 10, the digital imaging system 270 includes a line-scancamera 271, a lens 272, a light source 273, and a surface encoder 274.Line-scan camera 271 includes a one-dimensional array of photo sensors(not shown) that are spaced apart at precise intervals along a directionthat is perpendicular to media advance direction 208 (FIG. 11). Lightfrom light source 273 is transmitted through web of substrate 150 sothat features on both first side 151 and second side 152 are imagedthrough lens 272 and captured by line-scan camera 271. Surface encoder274 is in contact with a surface of the web of substrate 150 so that adistance that the web of substrate 150 has been advanced can bemeasured.

As illustrated in FIG. 11, when the surface encoder 274 indicates thatthe web of substrate 150 has been advanced in the media advancedirection 208 by a distance d that is nominally equal to the line widthof a single image line 275, the line-scan camera 271 (FIG. 10) iscontrolled to capture the next image line 275. In particular, thesurface encoder 274 provides an encoder signal representing a positionof the web of substrate 150, and a timing for capturing successive imagelines 275 is determined responsive to the encoder signal as the web ofsubstrate 150 moves past the line-scan camera 271. In this way theline-scan camera 271 of the digital imaging system 270 (FIG. 10)captures a set of successive image lines as the web of substrate 150 ismoved along the web transport path 205 (FIG. 10). The set of successiveimage lines 275 is combined by controller 260 to provide a compositedigital image that includes the test pads (connector pads 558, 568 andprobe pads 559, 569) as well as the associated fiducial marks 531, 532.A problem that can occur is that due to slippage of surface encoder 274or build-up of particulate contaminants on surface encoder 274, therecan be errors in measuring the media advance distance, and the captureof successive line images 275 by line-scan camera 271 can be thereforetriggered at wrong intervals. Because of this, distortion (such asstretching or shrinkage along the media advance direction 208) can beintroduced into the composite digital image analyzed by controller 260such that spatial relationships determined between positions of testpads (connector pads 558, 568 and probe pads 559, 569) and positions ofthe fiducial marks 531, 532 may become inaccurate. Therefore, it can bebeneficial to provide a method to calibrate the encoder signal from thesurface encoder 274, as will be discussed in more detail later.

In the example of electrical test system 210 described above withreference to FIG. 10, there is a first sensing system (fiducial sensingsystem 250) that is capable of determining the positions of the fiducialmarks 531, 532 with a high level of accuracy, but does not providedirect information about the actual positions of the test pads(connector pads 558, 568 and probe pads 559, 569); and a second sensingsystem (digital imaging system 270) that is capable of providingcomposite image that includes the fiducial marks 531, 532 as well as thetest pads (connector pads 558, 568 and probe pads 559, 569), but issusceptible to image distortion, especially along the media advancedirection 208. Typically, the fiducial sensing system 250 is locatedalong the web transport path 205 downstream of the digital imagingsystem 270 and includes a plurality of fiducial sensors 251, 252 havingassociated fields-of-view 253, 254 (FIG. 11) which are much smaller thanthe field-of-view associated with the digital imaging system 270. Insome embodiments of the invention, the fiducial mark positionsdetermined by the fiducial sensing system 250 are used together with thedigital image provided by the digital imaging system 270 to determinemore accurate estimates of the test pad positions and make appropriateadjustments in the position of the electrical test fixture 220. In suchembodiments, the very accurate distance measurements provided by thefiducial sensing system 250 can be used to compensate for geometricdistortion within the composite digital image and thereby to calibratethe digital imaging system 270.

FIG. 12 shows a flowchart of a method for using the improvedroll-to-roll electrical test system 210 of FIG. 10 to electrically testdevices fabricated on successive frames 300 on a web of substrate 150.Each frame 300 includes an associated plurality of fiducial marks 305(e.g., fiducial marks 531, 532) and an associated electrical device(e.g., touch sensor 530) having a series of test pads 310 (e.g.,connector pads 558, 568 and probe pads 559, 569). In a preferredembodiment, some or all of the steps of the method are performed byusing the controller 260 to control the appropriate components of theelectrical test system 210 and to perform associated analysisoperations.

In advance frame to digital imaging system step 315, the web ofsubstrate 150 is advanced along the web transport path 205 until aparticular frame 300 is proximate to the digital imaging system 270. Acapture digital image step 320 is used to capture a digital image 325using the digital imaging system 270. The digital image 325 includes atleast a portion of the particular frame 300 that includes the associatedfiducial marks 305 and the test pads 310 of the associated device.

An analyze digital image step 330 is then used to automatically analyzethe captured digital image 325 using controller 260 to determinefiducial mark positions 335 and test pad positions 340 within thecaptured digital image 325. The fiducial mark positions 335 and the testpad positions 340 can be determined and represented using any methodknown in the art. In exemplary embodiments, the fiducial mark positions335 and the test pad positions 340 are determined by detecting thecorresponding fiducial marks 305 and the test pads 310 using anyappropriate feature detection method. In an exemplary embodiment,centroids of the detected fiducial marks 305 and the test pads 310 arethen computed, and the x-y pixel positions of the centroids within thedigital image 325 are used to represent the corresponding fiducial markpositions 335 and the test pad positions 340. Methods for analyzing adigital image to detect fiducial marks 305 and the test pads 310 andmethods for computing corresponding centroids will be well-known tothose skilled in the art and any such method can be used in accordancewith the present invention. It should be noted that computing centroidsrepresent one example of identifying a position of a feature, but anyother appropriate means for identifying positions of the detectedfiducial marks 305 and test pads 310 can alternatively be used inaccordance with the present.

A determine spatial relationships step 345 is then used to determinespatial relationships 350 between the determined test pad positions 340of the test pads 310 and the determined fiducial mark positions 335 ofthe fiducial marks 305 in the captured digital image 325. The spatialrelationships 350 can be represented using any appropriate method knownin the art. In an exemplary embodiment, the fiducial mark positions 335are used to define a coordinate system. For example, if there are twofiducial marks 305, a first axis can be defined that passes through thedetermined fiducial mark positions 335, and a second axis can be definedwhich is orthogonal to the first axis and passes through one of thefiducial mark positions 335. If there are more than two fiducial marks,then a fitting process can be used to define the coordinate system thatrepresents the best fit to the determined fiducial mark positions 335.The coordinates of the test pad positions 340 in the coordinate systemcan then be determined and used as a representation of the spatialrelationships 350. In various embodiments, the coordinates can beexpressed in various units such as pixel coordinates or physicalcoordinates (e.g., millimeters). In some embodiments, a relativecoordinate system can be defined which is normalized by the distancebetween two of the fiducial mark positions 335. In another exemplaryembodiments, the spatial relationships 350 of the test pad positions 340relative to the fiducial mark positions 335 can be represented by offsetdistances (e.g., Δx and Δy) between the test pad positions 340 and oneor more of the fiducial mark positions.

In some cases, the optical system associated with the digital imagingsystem 270 (FIG. 10) may introduce certain geometrical distortions(e.g., “pincushion distortion” or “barrel distortion”) into the captureddigital image 325. Furthermore, when the digital imaging system 270 is aline-scan camera 271, any errors in the timing of successive image lines275 due to calibration errors for the surface encoder (e.g., due toparticulates on the surface encoder 274 or slippage of the surfaceencoder 274) can introduce additional geometric distortions. In someembodiments, any geometric distortions introduced by the digital imagingsystem 270 can be characterized (e.g., by capturing an image of a gridhaving a known geometry), and the captured digital image 325 can beadjusted to compensate for the geometric distortions as part of theprocess of determining the spatial relationships 350. The compensationcan be done as part of the analyze digital image step 330 or thedetermine spatial relationships step 345.

Next, an advance frame to test fixture step 360 is used to advance theweb of substrate 150 along the web transport path 205 (FIG. 10) untilthe frame 300 is proximate to the electrical test fixture 220. Asdiscussed earlier with reference to FIG. 10, the electrical test fixture220 includes a set of test probes 222, 224 adapted to make electricalcontact with corresponding test pads 310 of the associated device (e.g.,touch sensor 530 in FIG. 11). In a preferred embodiment, the web ofsubstrate 150 is stopped at a position where the test pads 310 would bealigned with the test probes 222, 224 if the device is positioned in itsnominal location on the web of substrate 150. Preferably, the web ofsubstrate 150 is clamped in a fixed position once it has stopped (e.g.,using clamps 245 in FIG. 10) to prevent any further movement of the webof substrate 150 until after the electrical test has been performed.

In sense fiducial marks step 365, the fiducial sensing system 250 (FIG.10) is used to sense positions of the fiducial marks 305 to determinesensed fiducial mark positions 370, preferably while the web ofsubstrate 150 is stopped. As discussed earlier, in an exemplaryembodiment the fiducial sensing system 250 includes a plurality offiducial sensors 251, 252 having corresponding fields-of-view 253, 254large enough to detect the fiducial marks 305 given the expected rangeof system variability. In some embodiments the fiducial sensors 251, 252are digital camera systems which capture digital images of the portionof the web of substrate including the fiducial marks 305. The sensedfiducial mark positions 370 can then be determined by automaticallyanalyzing the captured digital images. For example, the centroids of thefiducial marks 305 in the captured digital images can be determined andused to define the sensed fiducial mark positions 370. A calibrationprocess can be used during a system configuration stage to accuratelyrelate the pixel positions within the captured digital images to actualphysical locations within the electrical test fixture 220 (FIG. 10).

Next, an adjust test fixture position step 375 is used to adjust aposition of the electrical test fixture 220 (FIG. 10) responsive to thespatial relationships 350 between the positions of test pads 310 and thepositions of the fiducial marks 305 in the captured digital image 325,together with the sensed fiducial mark positions 370 so that the testprobes 222, 224 (FIG. 10) are properly aligned with the correspondingtest pads 310. In an exemplary embodiment, predicted positions of thetest pads 310 are determined by using the relative position informationspecified by the spatial relationships 350 to estimate the predictedpositions of the test pads 310 relative to the sensed fiducial markpositions 370. In order to increase the reliability of aligning the testprobes 222, 224 with corresponding test pads 310 it is advantageous forthe predicted position of each test pad 310 to correspond to a centralposition (e.g., a centroid) within the test pad 310. This can beaccomplished by computing centroids of the fiducial marks 305 and thetest pads 310 during the determination of the spatial relationships 350and the sensed fiducial mark positions.

The positions of the test probes 222, 224 are generally not individuallyadjustable but are moved as a unit. Therefore, once the predictedpositions of the test pads 310 are determined, the position of theelectrical test fixture 220 is adjusted to align the test probes 222,224 with the predicted positions of the test pads 310. In variousembodiments, the adjustment of the test fixture position can includetranslating the electrical test fixture 220 within a plane parallel to asurface of the web of substrate 150 (FIG. 10), as well as rotating theelectrical test fixture 220 about an axis that is perpendicular to asurface of the web of substrate 150.

In some embodiments, alignment parameters specifying the amount oftranslation and rotation that should be applied to the electrical testfixture are determined by using a fitting process (e.g., a well-knownleast squares analysis) based on the predicted positions of the testpads 310 and known positions of the test probes 222, 224 associated witha predefined test probe configuration within the electrical test fixture220. For example, the alignment parameters can include translationparameters specifying distances in the x- and y-directions (FIG. 9) formoving the electrical test fixture 220, and a rotation parameterspecifying an angle θ for rotating the electrical test fixture 220. Theadjust test fixture position step 375 then adjusts the position of theelectrical test fixture 220 responsive to the determined alignmentparameters. In some embodiments, the positions of the upper test bed 221and the lower test bed 223 of the electrical test fixture 220 can beindependently adjusted. In such cases, the alignment parameters caninclude separate sets of alignment parameters for adjusting thepositions of the upper test bed 221 and the lower test bed 223.

A move test probes to contact test pads step 380 is used to move thetest probes 222, 224 (FIG. 10) into electrical contact with thecorresponding test pads 310. In some embodiments this is accomplished bylowering the upper test bed 221 (FIG. 10) until the associated testprobes 222 come into electrical contact with the test pads 310 on thefirst side 151 of the web of substrate 150. Likewise, the lower test bed223 can be raised until the associated test probes 224 come intoelectrical contact with the test pads 310 on the second side 152 of theweb of substrate 150. In other cases, different mechanisms can be usedto extend the test probes 222, 224 into electrical contact with thecorresponding test pads 310.

In perform electrical test step 385 an electrical test of the device(s)contained within the frame 300 is performed using the test instrument240 (FIG. 4) associated with the set of test probes 222, 224 (FIG. 10).In some embodiments, the electrical test can include verifying thatthere is electrical connectivity between appropriate pairs of test pads310, and that there are no undesired shorts between other pairs of testpads 310. In other embodiments, electrical characteristics such asresistance, capacitance or inductance can be measured between pairs oftest pads 310 to make sure that they fall within expected ranges. Insome cases, performing the electrical test can include applying anappropriate electrical signal (e.g., a voltage or a current) to one ormore of the test pads 310, and measuring a corresponding response atother test pads 310.

After the electrical test step 385 is completed, the results can berecorded using an appropriate means (e.g., by storing the results inmemory 265). The web of substrate 150 can then be advanced to test thedevices on the next frame 300. The steps of FIG. 12 can then be repeatedfor each successive frame 300 on the web of substrate 150.

Returning to a discussion of FIG. 11, a top view of three frames 300 a,300 b and 300 c is illustrated, where frame 300 a has already reachedelectrical test fixture 220 and is positioned so that its fiducial marks531, 532 are within the fields-of-view 253, 254 of fiducial sensors 251,252 (FIG. 10), so that their positions can be sensed by the fiducialsensing system 250 (FIG. 10). Frame 300 c is proximate digital imagingsystem 270 (FIG. 10) and is positioned for the line-scan camera 271 tosequentially capture line images 275 as frame 300 c is advanced past theline-scan camera 271 along media advance direction 208. Frame 300 b isintermediate in position between digital imaging system 270 (FIG. 10)and electrical test fixture 220. Line images 275 for frames 300 a and300 b were previously captured when they passed through the currentposition of frame 300 c.

Note in frame 300 a that fiducial mark 531 is spaced apart from fiducialmark 532 along the media advance direction 208, so that the fiducialsensing system 250 can make an accurate determination of the actualdistance D1 between fiducial marks 531 and 532. In some embodiments, thevalue of D1 determined by the fiducial sensing system 250 can be used tocalibrate the encoder signal from the surface encoder 274. Anyinaccuracies in the calibration of the encoder signal will result in adifference between the actual distance D1 between the fiducial marks 531and 532 determined by the fiducial sensing system 250 and acorresponding distance between the fiducial marks 531 and 532 in thedigital image captured by the line-scan camera 271. The ratio of thedistances determined by the fiducial sensing system 250 and theline-scan camera 271 can be used to provide a scale factor for adjustingthe calibration of the surface encoder 274 so that the distance d thatthe web of media 150 is advanced between capturing sequential imagelines 275 will be more accurate.

For purposes of illustration of both types of optical sensing, FIG. 11shows the fields-of-view 253, 254 of the fiducial sensors 251, 252centered around the fiducial marks 531, 532, respectively, for frame 300a at the same time that line images 275 for frame 300 c are beingcaptured by the line-scan camera 271. However, in practice, the fiducialmarks 531, 532 will be centered within the fields-of-view 253, 254 whenthe web of substrate 150 is stopped and clamped by clamps 245 (FIG. 10).In contrast, the line images 275 will be captured by the line-scancamera 271 when frame 300 c on the web of substrate 150 is moving pastthe line-scan camera 271. In an exemplary embodiment, a distance isprovided between the line-scan camera 271 and the fiducial sensingsystem 250 (FIG. 10) such that when the fiducial marks 531, 532 of aparticular frame (e.g., frame 300 a) are proximate the associatedfiducial sensors 251, 252 and the web of substrate 150 is stopped, afield-of-view of the line-scan camera 271 is positioned at aninter-frame region between two successive frames (such as frames 300 band 300 c). The inter-frame region can include a frame boundary 546,and, in general, does not include the fiducial marks 531, 532 orportions of the touch sensor 530 for any of the successive frames. Inthis way, the capturing of the digital image 325 (FIG. 12) by thedigital imaging system 270 is not interrupted when the web of substrate150 is stopped.

In a preferred embodiment, the fiducial sensors 251, 252 of the fiducialsensing system 250 (FIG. 10) are digital camera systems, where thefields-of-view 253, 254 of the digital camera systems are larger than anexpected variability in the position of the corresponding fiducial marks531, 532. Additionally, in a preferred embodiment, each of digitalcamera systems has a field-of-view that is smaller than a field-of-viewof the digital imaging system 270. (For the case where the digitalimaging system 270 uses a line-scan camera 271, the “field-of-view” ofthe digital imaging system 270 can be considered to correspond to thearea of the composite digital image formed by combining a plurality ofimage lines rather than the field-of-view of the line-scan camera 271,which would correspond to a particular image line 275.) Thefield-of-view of the digital imaging system 270 is preferably arrangedto include at least a portion of a particular frame 300 c that includesthe test pads (connector pads 558, 568 and probe pads 559, 569) of theparticular device together with the fiducial marks 531, 532. One reasonthat the fields-of-view 253, 254 of the fiducial sensors 251, 252 willgenerally be relatively small is that the components of the electricaltest fixture 220 will typically block the central region of the frame300 a that includes the device (i.e., touch sensor 530) from view.

The examples above have described the use of two fiducial sensors 251,252 to determine the positions of two fiducial marks 531, 532 that arespaced apart along the media advance direction 208. In otherembodiments, three or more fiducial sensors can be used to determine thepositions of a corresponding set of fiducial marks. A series of three ormore fiducial marks can be positioned along a straight line within theframe 300 in order to sense and compensate for distortion at a morelocal scale within the frame using a corresponding series of fiducialsensors. In another implementation of three or more fiducial marks(e.g., fiducial marks 531, 532, 533 and 534), the fiducial marks andcorresponding fiducial sensors are not arranged along a straight line,and can be used to compensate for distortion along more than onedirection. This will enable more accurate determination of anydistortions introduced into the web of media 150 which can cause thefeatures of the device (i.e., the touch sensor 530) to be moved fromtheir nominal positions. In turn, this will enable more accuratedetermination of predicted test pad positions, in both the in-trackdirection (i.e., the x-direction in FIG. 9) and the cross-trackdirection (i.e., the y-direction in FIG. 9).

For testing of devices such as touch sensors 530 having a first set oftest pads (connector pads 558 and probe pads 559) that are printed on afirst side 151 of web of substrate 150 by a first print module 130 offlexographic printing system 100 (FIG. 1) and a second set of test pads(connector pads 568 and probe pads 569) that are printed on a secondside 152 of web of substrate 150 by a second print module 140 offlexographic printing system 100, it is preferable to have a first setfiducial marks 531, 532 printed by the first print module 130 on thefirst side 151, and a second set of fiducial marks 533, 534 printed bythe second print module 140 on the second side 152. This requires twoadditional fiducial sensors (not shown in FIG. 10), but it eliminates asource of error that would be introduced if the fiducial marks used todetermine predicted test pad positions for the test pads printed on aparticular side (e.g., second side 152) were not printed by the sameprinting station that is used to print the test pads (e.g., connectorpads 568 and probe pads 569) on that side of the web of substrate 150.

In the examples described above, each frame 300 has contained only onetouch sensor 530. In other embodiments, a single frame 300 can include aplurality of touch sensors 530 depending upon the size of an individualdevice relative to the size of a frame. FIG. 13 shows an example where asingle frame 300 contains four touch sensors 530 a, 530 b, 530 c, 530 d.The width W of a frame 300 is typically the width of the web ofsubstrate 150, which can be around 14 inches, although narrower webs andwider webs are also possible. The length L of a frame 300 that has beenprinted using flexographic printing system 100 (FIG. 1) generallycorresponds to the circumference of a plate cylinder, such as platecylinder 111, which can be around 18 inches. Depending upon the size ofthe touch sensor 530, it can be cost effective to have a plurality oftouch sensors within each frame 300.

In the example shown in FIG. 13, probe pads 559 are spaced at a firstspacing S1 and connector pads 558 are spaced at a second spacing S2 thatis less than S1. Similarly connector pads 568 are spaced at a smallerspacing than that of probe pads 569. This is because it is desired touse a small connector (not shown) when the touch sensor 530 isintegrated into a touch screen. Connector pads 558 and 568 are typicallylocated at or near an edge 535 of the touch sensors 530 a, 530 b, 530 c,530 d to facilitate attachment of the connector.

When all four touch sensors 530 a, 530 b, 530 c, 530 d in frame 300 areoriented in the same orientation as in FIG. 13, some tightly spacedconnector pads 558, 568, such as those associated with touch sensors 530a and 530 b, can be located comparatively close to the fiducial marks531, 532 on the first side 151 of the web of substrate 150 andcomparatively far from the fiducial marks 533, 534 on the second side152 of the web of substrate 150. Since the accuracy of the determinationof the predicted test pad positions will be related to their distancefrom the corresponding fiducial marks, this can result in some of thepredicted test pad positions having a significantly lower level ofaccuracy. The accuracy in the determination of the predicted test padpositions of the tightly spaced connector pads 558, 568 can be improvedby configuring the layout of the touch sensors 530 a, 530 b, 530 c and530 d as in FIG. 14, where the tightly spaced connector pads 558, 568for touch sensors 530 a and 530 b are near the tightly spaced connectorpads 558, 568 for touch sensors 530 c and 530 d, both being positionedroughly halfway across the width of web of substrate 150. This can beachieved by laying out the flexographic printing plates for theflexographic printing system 100 (FIG. 1) such that the orientation oftouch sensors 530 a and 530 b is rotated by 180 degrees relative to theorientation of touch sensors 530 c and 530 d.

In some cases, the distortion of the web of substrate 150 may be suchthat it would not be possible to simultaneously align all of the testprobes 222, 224 with their corresponding test pads (e.g., connector pads558, 568 and probe pads 559, 569). In this case, it may be desirable toalign the electrical test fixture 220 with one of the touch sensors 530a, 530 b, 530 c, 530 d at a time. For example, the test probes 222, 224can first be aligned with the test pads (e.g., connector pads 558, 568and probe pads 559, 569) of touch sensor 530 a so that that device canbe tested. The position of the electrical test fixture 220 can then beadjusted to align with the test pads of touch sensors 530 b, 530 c and530 d so that they can be tested in sequence. In this way, each of thetouch sensors 530 a, 530 b, 530 c, 530 d can be tested individuallydespite the fact that they could not be tested simultaneously.

Further improvements in reliability of positioning the electrical testfixture 220 relative to the test pads (e.g., connector pads 558, 568 andprobe pads 559, 569) can make use of an adaptive learning process tocompensate for errors in the systematic deviations in the positions ofindividual test probes 222, 224 (FIG. 10). For example, in a set-upprocess, the centroids of the test pads can be determined and theelectrical test fixture can be moved to a first position correspondingto an ideal position that would cause each test probe 222, 224 tocontact the corresponding test pad at its centroid, assuming zero errorin the positioning of the individual test probes 222, 224. In this firststep it can be confirmed that there are no electrical defects. Then theelectrical test fixture 220 can be moved by a distance that is lessthan, but approximately equal to, half the width of a narrow test pad(e.g., a connector pad 558) in the +x direction to check whether anytest probes 222 or 224 have lost contact with the corresponding testpad. Similar tests can be done using small movements in the −xdirection, the +y direction, and the −y direction. Through such anadaptive learning process, it can be determined which particular testprobes 222, 224 have the greatest positional error in the variousdirections and how large the errors are. A modified amount of motion ofthe electrical test fixture 220 can thereby be determined such thatreliable contact of the overall set of test probes 222, 224 can beestablished, taking into account the systematic errors in the test probepositions, as well as image distortion and web distortion.

After the electrical test is performed at perform electrical test step385 (FIG. 12), the result of the electrical test (for example, “pass” or“fail”) needs to be coupled with the corresponding device that wastested. One method of coupling the test result with the correspondingdevice is shown in FIG. 15A. In this case, a physical mark 390 is formedon the web of substrate 150 proximate to the device (e.g., touch sensor530) providing an indication of whether the device passed or failed theelectrical test. The physical mark 390 can be formed using any processknown in the art, such as applying ink to the web of substrate 150, orusing a laser to burn a mark on the web of substrate 150 or to form ahole in the web of substrate 150. The physical mark 390 can subsequentlybe detected for separating the good and bad devices. In someembodiments, no mark can be formed if the device passes the electricaltest, and a mark can be formed if the device fails the electrical test(or vice versa). In other embodiments, different marks can be formeddepending on whether or not the device passes the electrical test.

Alternatively, a unique identification code can be associated with eachdevice. In this case, the step of coupling each device with the resultof the electrical test can include storing an electrical test result inmemory 265 (FIG. 10), together with the identification code for thatdevice. FIG. 15B shows an example where the identification codeassociated with each device is represented using a bar code 395 formedon the web of substrate 150 in proximity to the device (e.g., touchsensor 530). In other cases, the identification code can simply providean indication of a location of the device on the web of media 150, andindication of the identification code may not be physically formed ontothe web of media 150.

As indicated above, exemplary embodiments of the invention include aroll-to-roll electrical testing system and associated method where anelectrical test fixture is aligned to test pads formed on a web ofsubstrate 150 using a digital imaging system 270 for capturing a digitalimage of at least the test pads of a particular device (e.g., touchsensor 530) plus associated fiducial marks 531, 532 in order toestablish their spatial relationships, and also using a separatefiducial sensing system 250 for sensing positions of the associatedfiducial marks 531, 532. Analogous types of systems and methods can beused for performing various other types of operations on an articlehaving a plurality of fiducial marks and a set of features as describedbelow.

FIG. 16 shows a top view of an article 600 having fiducial marks 601,602 and a set of probe pads 615 (which can generically be called a setof “features”) connected to an array of fuses 610 that are associatedwith a circuit 618. In some embodiments, the state of fuses 610 (open orshorted) can affect the operation of circuit 618. In other embodiments,the state of the fuses 610 can serve as an electrically readable code.In an exemplary embodiment, the fuses 610 are selectively “blown” toconfigure the article 600 to have a desired state (e.g., to store adesired code). Subsequently, it can be useful to electrically read thestate of the fuses (e.g., to read the code). In either case, a system630 such as that shown in FIG. 17, which is similar to the electricaltest system 210 (FIG. 10), can be used. While the system 630 includes aweb transport system including a supply roll 202, a take-up roll 204,and rollers 206 and 207, these components would only be included in sucha system 630 if the article 600 is formed on a flexible web. In otherembodiments (not shown) the article 600 can be formed on a rigidsubstrate and would require appropriate transport system components fortransporting and positioning the article 600 within the system 630.Examples of such transport systems would include a conveyor belt withvacuum hold down or electrostatic hold down, or a substrate holder withmechanical clamps for holding the article 600 in a fixed positionrelative to the transport system.

With reference to FIGS. 16 and 17, an exemplary embodiment will bedescribed where the system 630 is used to perform the operations ofblowing or testing fuses 610. Digital imaging system 270 is configuredto capture a digital image of at least a portion of the article 600 thatincludes a set of features (e.g., the probe pads 615) and the fiducialmarks 601, 602, and fiducial sensing system 250 is configured to sensepositions of the fiducial marks 601, 602. An instrument 635 isconfigured to perform the desired operation on the article 600, andincludes both upper test bed 221 and lower test bed 223 with respectivetest probes 222, 224, as well as an electrical fixture 640 adapted tocontrollably apply electrical signals through the test probes 222, 224to perform the desired operations. For example, to selectively blow thefuses 610, the electrical fixture 640 can controllably applysufficiently high voltages across the fuses 610 through the probe pads615. Similarly, to test the state of the fuses 610, the electricalfixture 640 can be used to test the electrical continuity between pairsof probe pads 615 as in the embodiments for roll-to-roll testing oftouch sensors 530 described above. Controller 260 is configured tocontrol the components of the system 630 to perform the desiredoperation(s).

Other aspects of the system 630 for performing an operation on anarticle can be similar to those in embodiments described above relativeto the roll-to-roll electrical test system 210 described relative toFIGS. 10 and 11. For example, in some embodiments, the digital imagingsystem 270 can include a line-scan camera 271 configured to capture aset of successive image lines 275 as the article 600 is moved past theline-scan camera 271, such that the set of successive image lines arecombined to provide a composite digital image. The digital imagingsystem 270 can also include a surface encoder 274 that is configured tocontact a surface of the article 600 and provide an encoder signalrepresenting a position of the article 600, where a timing for capturingthe successive image lines 275 is determined responsive to the encodersignal. As described earlier, in some embodiments, a difference betweensensed positions of the fiducial marks 601, 602 by the fiducial sensingsystem 250 can be used to calibrate the encoder signal.

Other aspects of the system 630 that can be similar to the electricaltest system 210, include the use of a plurality of digital camerasystems in the fiducial sensing system; the field-of-view size beinglarger than the expected variability in the position of the fiducialmark; and the field-of-view of the digital camera systems being smallerthan the field-of-view of the digital imaging system 270.

Some aspects of the generalized system may also be different, dependingupon the nature of the article 600 as well as the way the generalizedsystem functions. In the roll-to-roll electrical test system 210described with reference to FIGS. 10 and 11, the web transport systemadvances the web of substrate 150 along a media advance direction 208.In some embodiments of the generalized system, it is necessary to movethe article 600 in a forward direction and then in a reverse direction.For example, for testing and trimming the resistor array shown below inFIG. 19, article 600 can be moved forward from an electrical testingstation to a laser trimming station and then backward to the electricaltesting station for a post-trimming electrical test. In such anembodiment, errors in the reading of the surface encoder 274 can beincreased due to slippage or play during the reversal of direction. Thiscan increase the need for a fiducial sensing system 250 to accuratelyposition that article for each operation and to provide calibration ofthe digital imaging system. Another difference relative to electricaltest system 210 of FIG. 10 is that if article 600 is not transparent, achange in the placement of light source 273 can be required.

One further difference in the generalized system 630 is that adjustmentof position (such as translation or rotation) can either be done for theinstrument 635 or for the article 600, depending on the nature of thearticle 600 and the configuration of the system 630. In someembodiments, translation of the article 600 can be done on a conveyorbelt or a translation stage (not shown), and rotation of the article 600can be done using a rotational stage (not shown).

A generalized method for using an instrument 635 to perform an operationon an article 400 including a plurality of fiducial marks 405 and a setof features 410 is described relative to the system 630 of FIG. 17 andthe flow diagram shown in FIG. 18. In an exemplary embodiment, some orall of the steps of the method are performed by using the controller 260to control the appropriate components of the system 630 and to performassociated analysis operations.

In position article proximate to digital imaging system step 415, thearticle 400 is positioned in proximity to the digital imaging system270. In the illustrated roll-to-roll configuration, this step can beperformed by advancing the web of substrate 150 along the web transportpath 205 until a particular article 400 is proximate to the digitalimaging system 270. A capture digital image step 420 is used to capturea digital image 425 using the digital imaging system 270. The digitalimage 425 includes at least a portion of the article 400 that includesthe associated fiducial marks 405 and the features 410. In theillustrated configuration, the digital imaging system 270 includes aline-scan camera 271 which captures the digital image 425 line-by-lineas the article 400 is moved past the line-scan camera 271. In otherembodiments, the digital imaging system 270 can use a two-dimensionaldigital camera.

An analyze digital image step 430 is then used to automatically analyzethe captured digital image 425 using controller 260 to determinefiducial mark positions 435 and feature positions 440 within thecaptured digital image 425. The fiducial mark positions 435 and thefeature positions 440 can be determined and represented using any methodknown in the art. In exemplary embodiments, the fiducial mark positions435 and the feature positions 440 are determined by detecting thecorresponding fiducial marks 405 and the features 410 (e.g., probe pads615) using any appropriate feature detection method. Centroids of thedetected fiducial marks 405 and the features 410 are then computed, andthe x-y pixel positions of the centroids within the digital image 425are used to represent the corresponding fiducial mark positions 435 andthe feature positions 440. Methods for analyzing a digital image todetect the fiducial marks 405 and the features 410, and methods forcomputing corresponding centroids will be well-known to those skilled inthe art and any such method can be used in accordance with the presentinvention.

A determine spatial relationships step 445 is then used to determinespatial relationships 450 between the determined feature positions 440of the features 410 and the determined fiducial mark positions 435 ofthe fiducial marks 405 in the captured digital image 425. As describedearlier with reference to FIG. 12, the spatial relationships 450 can berepresented using any appropriate method known in the art. For caseswhere the optical system associated with the digital imaging system 270introduces geometrical distortions into the captured digital image 425,the captured digital image 425 can be adjusted to compensate for thegeometric distortions as part of the process of determining the spatialrelationships 350. The compensation can be done as part of the analyzedigital image step 430 or the determine spatial relationships step 445.

Next, a position article proximate to instrument step 460 is used toreposition the article 400 so that it is proximate to the instrument 635which will perform the operation on the article 400. In the illustratedroll-to-roll configuration, this step can be performed by advancing theweb of substrate 150 along the web transport path 205 until the article400 is proximate to the instrument 635. As discussed earlier withreference to FIG. 10, the exemplary instrument 635 includes a set oftest probes 222, 224 adapted to make electrical contact withcorresponding features 410 (e.g., probe pads 615) of the associatedarticle 400. In a preferred embodiment, the web of substrate 150 isstopped at position where the features 410 would be aligned with thetest probes 222, 224 if the article 400 is positioned in its nominallocation on the web of substrate 150. Preferably, the web of substrate150 is clamped in a fixed position once it has stopped (e.g., usingclamps 245) to prevent any further movement of the web of substrate 150until after the operation has been performed.

In sense fiducial marks step 465, the fiducial sensing system 250 isused to sense positions of the fiducial marks 405 to determine sensedfiducial mark positions 470, preferably while the article 400 is in afixed position. As discussed earlier, in an exemplary embodiment thefiducial sensing system 250 includes a plurality of fiducial sensors251, 252 having corresponding fields-of-view 253, 254 large enough todetect the fiducial marks 405 given the expected range of systemvariability. In some embodiments the fiducial sensors 251, 252 aredigital camera systems which capture digital images of the portion ofthe web of substrate including the fiducial marks 405. The sensedfiducial mark positions 470 can then be determined by automaticallyanalyzing the captured digital images. For example, the centroids of thefiducial marks 405 in the captured digital images can be determined andused to define the sensed fiducial mark positions 470. A calibrationprocess can be used during a system configuration stage to accuratelyrelate the pixel positions within the captured digital images to actualphysical locations within the instrument 635.

A determine predicted feature positions step 475 is used to determinedpredicted feature positions 480 of the features 410 using the relativeposition information specified by the spatial relationships 450 toestimate the predicted positions of the features 410 relative to thesensed fiducial mark positions 470. In an exemplary embodiment, thepredicted feature positions 480 correspond to central position (e.g., acentroid) within the features 410.

Next, an adjust position of article or instrument step 485 is used toadjust a position of the instrument 635 or the article 400 responsive tothe predicted feature positions 480 so that the instrument 635 isproperly positioned to perform the operation on the article 400. In anexemplary embodiment, the position of the instrument 635 is adjusted sothat the test probes 222, 224 are properly aligned with thecorresponding features 410 (e.g., probe pads 615).

In an exemplary embodiment, the elements of the instrument 635 (e.g.,test probes 222, 224) are not individually adjustable but must be movedas a unit. Therefore, once the predicted positions of the features 410are determined, the position of the instrument 635 is adjusted to alignthe test probes 222, 224 with the predicted positions of the features410. In various embodiments, the adjustment of the position of theinstrument 635 can include translating the instrument 635 within a planeparallel to a surface of the web of substrate 150, as well as rotatingthe instrument 635 about an axis that is perpendicular to a surface ofthe web of substrate 150.

In some embodiments, alignment parameters specifying the amount oftranslation and rotation that should be applied to the instrument aredetermined by using a fitting process (e.g., a well-known least squaresanalysis) based on the predicted feature positions 480 and knownpositions of the test probes 222, 224 associated with a predefined testprobe configuration within the instrument 635. For example, thealignment parameters can include translation parameters specifyingdistances in the x- and y-directions (FIG. 9) for moving the instrument635, and a rotation parameter specifying an angle θ for rotating theinstrument 635. The adjust position of article or instrument step 485then adjusts the position of the instrument 635 responsive to thedetermined alignment parameters. In some embodiments, the positions ofvarious components of the instrument 635 (e.g., the upper test bed 221and the lower test bed 223) can be independently adjusted. In suchcases, the alignment parameters can include separate sets of alignmentparameters for adjusting the positions of the different components.

In perform operation step 490, the desired operation is performed on thearticle 400 using the instrument 635. As described earlier with respectto FIG. 16, in an exemplary embodiment the operation can includebringing the test probes 222, 224 into contact with the probe pads 615and applying appropriate electrical signals through the probe pads 615to selectively blow the fuses 610 or to detect the state of the fuses610.

In other embodiments, the operation performed by the perform operationstep 490 can include adding material to the article 400 using adeposition process or a printing process, for example. In suchembodiments there would generally be no test probes 222, 224 to contactthe article 400. Rather, the instrument 635 would include one or moredeposition or printing devices, such as ink jetting devices that arearranged at predetermined positions. In this case, particular features410 on article 400 would serve as local alignment targets for materialdeposition or printing, while fiducial marks 405 serve as globalposition reference marks. The printing or deposition of material can befor preparing images to be viewed, or for fabricating functionaldevices, or for forming three dimensional patterns, for example.Alignment of the deposition or printing devices to article 400 isperformed as described above using digital imaging system 270 forcapturing a digital image of at least the features 410 of a particulararticle 400 plus associated fiducial marks 405, and a separate fiducialsensing system 250 for sensing positions of the associated fiducialmarks 405.

In still other embodiments, the operation performed by the performoperation step 490 can include removing material from the article 400.In various embodiments material removal can be performed using anoperation such as an etching process, an ablation process, a drillingprocess, a milling process or a cutting process.

In still other embodiments, the operation performed by the performoperation step 490 can include cutting the article 400 into multiplepieces (e.g., using a milling operation or a cleaving operation) orattaching the article 400 to another object (e.g., using a gluingoperation or a welding operation).

FIG. 19 shows a top view of an exemplary article 600 including an arrayof resistors 620 with associated probe pads 615, as well as fiducialmarks 601, 602. The resistors 620 can be formed by thick film screenprinting on a rigid substrate or flexographically printed on a web ofsubstrate 150 (FIG. 1), for example. Variations in resistance values ofthe resistors 620 can occur due to printing variations or materialvariations, for example. The resistance values can be brought to theirtarget values by controllably removing resistor material in a processknown as resistor trimming. Material can be removed, for example, bylaser ablation or by controllably directing jets of abrasive material atthe resistors 620. In a first operation, the initial resistance valuescan be determined via an electrical test where test probes are alignedwith probe pads 615 (using adjust position of article or instrument step485 as described above), and the electrical fixture 640 can apply aknown voltage to the probe pads 615 and measure the resulting current.In a second operation, at least one laser or at least one abrasive jetcan be aligned with selected resistors 620 (using adjust position ofarticle or instrument step 485 as described above), and the resistors620 can be trimmed to provide the aim resistance values.

The invention has been described in detail with particular reference tocertain preferred embodiments thereof, but it will be understood thatvariations and modifications can be effected within the spirit and scopeof the invention.

PARTS LIST

-   100 flexographic printing system-   102 supply roll-   104 take-up roll-   105 roll-to-roll direction-   106 roller-   107 roller-   110 print module-   111 plate cylinder-   112 flexographic printing plate-   113 raised features-   114 impression cylinder-   115 anilox roller-   116 UV curing station-   120 print module-   121 plate cylinder-   122 flexographic printing plate-   124 impression cylinder-   125 anilox roller-   126 UV curing station-   130 print module-   131 plate cylinder-   132 flexographic printing plate-   134 impression cylinder-   135 anilox roller-   136 UV curing station-   140 print module-   141 plate cylinder-   142 flexographic printing plate-   144 impression cylinder-   145 anilox roller-   146 UV curing station-   150 substrate-   151 first side-   152 second side-   200 electrical test system-   202 supply roll-   204 take-up roll-   205 web transport path-   206 roller-   207 roller-   208 media advance direction-   210 electrical test system-   220 electrical test fixture-   221 upper test bed-   222 test probes-   222 a test probes-   222 b test probes-   223 lower test bed-   224 test probes-   224 a test probes-   224 b test probes-   240 test instrument-   245 clamp-   250 fiducial sensing system-   251 fiducial sensor-   252 fiducial sensor-   253 field-of-view-   254 field-of-view-   260 controller-   265 memory-   270 digital imaging system-   271 line-scan camera-   272 lens-   273 light source-   274 surface encoder-   275 image line-   300 frame-   300 a frame-   300 b frame-   300 c frame-   305 fiducial marks-   310 test pads-   315 advance frame to digital imaging system step-   320 capture digital image step-   325 digital image-   330 analyze digital image step-   335 fiducial mark positions-   340 test pad positions-   345 determine spatial relationships step-   350 spatial relationships-   360 advance frame to test fixture step-   365 sense fiducial marks step-   370 sensed fiducial mark positions-   375 adjust test fixture position step-   380 move test probes to contact test pads step-   385 perform electrical test step-   390 physical mark-   395 bar code-   400 article-   405 fiducial marks-   410 features-   415 position article proximate to digital imaging system step-   420 capture digital image step-   425 digital image-   430 analyze digital image step-   435 fiducial mark positions-   440 feature positions-   445 determine spatial relationships step-   450 spatial relationships-   460 position article proximate to instrument step-   465 sense fiducial marks step-   470 sensed fiducial mark positions-   475 determine predicted feature positions step-   480 predicted feature positions-   485 adjust position of article or instrument step-   490 perform operation step-   500 apparatus-   510 touch screen-   520 display device-   530 touch sensor-   531 fiducial mark-   532 fiducial mark-   533 fiducial mark-   534 fiducial mark-   535 edge-   540 transparent substrate-   541 first side-   542 second side-   543 first edge-   544 second edge-   546 frame boundary-   550 conductive pattern-   551 fine lines-   552 grid-   553 fine lines-   554 channel pads-   555 grid column-   556 interconnect lines-   558 connector pads-   559 probe pads-   560 conductive pattern-   561 fine lines-   562 grid-   563 fine lines-   564 channel pads-   565 grid row-   566 interconnect lines-   568 connector pads-   569 probe pads-   580 touch screen controller-   600 article-   601 fiducial mark-   602 fiducial mark-   610 fuses-   615 probe pads-   618 circuit-   620 resistor-   630 system-   635 instrument-   640 electrical fixture-   D1 spacing-   D2 spacing-   θ angle

1. A method for using an instrument to perform an operation on anarticle, the article including a plurality of fiducial marks and a setof features, comprising: a) positioning the article in proximity to adigital imaging system; b) using the digital imaging system to capture adigital image of at least a portion of the article that includes the setof features and the plurality of fiducial marks; c) automaticallyanalyzing the captured digital image to determine positions of theplurality of fiducial marks and the set of features within the captureddigital image; d) determining spatial relationships between thedetermined positions of the plurality of fiducial marks and thedetermined positions of the set of features in the captured digitalimage; e) positioning the article in proximity to the instrument; f)using a fiducial sensing system to sense positions of the plurality offiducial marks; g) determining predicted positions of the featuresresponsive to the determined positions of the plurality of fiducialmarks and the determined positions of the set of features in thecaptured digital image and the sensed positions of the plurality offiducial marks; h) adjusting a position of the instrument or the articleresponsive to the predicted positions of the features; and i)controlling the instrument to perform the operation on the article. 2.The method of claim 1, wherein the article is an electrical device, thefeatures are probe pads, and the instrument includes an electricalfixture adapted to perform an electrical operation on the electricaldevice, the electrical fixture having a set of probes adapted to makeelectrical contact with the probe pads, and wherein the adjustment ofthe position of the instrument or the article aligns the probes withcorresponding probe pads.
 3. The method of claim 2, wherein theelectrical fixture is an electrical test fixture adapted to perform anelectrical test of the electrical device.
 4. The method of claim 2,wherein the electrical fixture is adapted to apply an electrical signalto the electrical device through the probe pads.
 5. The method of claim1, wherein the operation performed by the instrument includes addingmaterial to the article.
 6. The method of claim 5, wherein the materialis added to the article using a deposition process or a printingprocess.
 7. The method of claim 1, wherein the operation performed bythe instrument includes removing material from the article.
 8. Themethod of claim 7, wherein the material is removed from the articleusing an etching process, an ablation process, a drilling process, amilling process or a cutting process.
 9. The method of claim 1, whereinthe digital imaging system includes a line-scan camera configured tocapture a set of successive image lines as the article is moved along atransport path in a transport direction, wherein the set of successiveimage lines are combined to provide the digital image.
 10. The method ofclaim 9, wherein the digital imaging system further includes an encoderconfigured to contact a surface of the article and provide an encodersignal representing a position of the article, and wherein a timing forcapturing the successive image lines is determined responsive to theencoder signal.
 11. The method of claim 10, wherein a difference betweenthe sensed positions of the fiducial marks is used to calibrate theencoder signal.
 12. The method of claim 9, wherein the article is on aweb of substrate, and wherein the transport path is a web transportpath.
 13. The method of claim 1, wherein the fiducial sensing systemincludes a plurality of digital camera systems, each adapted to sensethe position of a corresponding fiducial mark.
 14. The method of claim13, wherein each of the digital camera systems has a field-of-view thatis larger than an expected variability in the position of thecorresponding fiducial mark.
 15. The method of claim 13, wherein each ofthe digital camera systems has a field-of-view that is smaller than afield-of-view of the digital imaging system.
 16. The method of claim 1,wherein the step of adjusting the position of the instrument includestranslating the instrument within a plane parallel to a surface of thearticle.
 17. The method of claim 1, wherein the step of adjusting theposition of the instrument includes rotating the instrument about anaxis that is perpendicular to a surface of the article.
 18. The methodof claim 1, wherein the step of adjusting the position of the articleincludes translating or rotating the article.
 19. The method of claim 1,wherein the digital imaging system introduces geometric distortions intothe captured digital image, and wherein the step of determining thespatial relationships between the determined positions of the pluralityof fiducial marks and the determined positions of the set of features inthe captured digital image includes compensating for the geometricdistortions.